The subject invention relates to the field of optical metrology, particularly broadband optical metrology tools for performing measurements of patterned thin films on semiconductor integrated circuits.
A number of metrology tools are now available for performing optical measurements on semiconductors. Such tools can include spectrophotometers, spectroscopic reflectometers, spectroscopic ellipsometers and spectroscopic scatterometers. Examples of such tools can be found in U.S. Pat. Nos. 5,608,526 and 6,278,519, both incorporated by reference. Such tools typically include a broadband light source for generating a probe beam that is directed to reflect off the sample. Changes in intensity or polarization state of the beam as a function of wavelength are monitored to yield information about the sample.
Given the continuing shrinking feature size of semiconductor circuits, it is desirable to design the illumination system to provide a tightly focused probe beam to form a small spot on the sample surface. For patterned samples, such as integrated circuits, the metrology instrument must measure within small test features [i.e. often less than 50 microns wide] surrounded by a completely different material or film stack. Several focusing assemblies have been developed for this purpose. These focusing assemblies can be formed from refractive or reflective elements or a combination of each (cadiatropic assemblies).
It is a common practice to use reflective objective designs in broadband, DUV to IR optical systems. Reflective designs are attractive since they have no chromatic aberration; however off-axis aberrations limit the field of view. Furthermore, reflective optics are more sensitive to manufacturing and alignment errors and, the polarization changes that occur upon reflection from the optical surfaces can become a source of measurement error in ellipsometric applications.
In the prior art, rotationally symmetric reflective objective designs [e.g. the Schwarzchild microscope] are common. This design contains a central obscuration that blocks the light near zero numerical aperture and increases diffractive effects, limiting the size of the focal spot. Prior art, off-axis reflective objective designs may avoid the central obscuration problem of the rotationally symmetric objective. However, the only cost effective method for fabricating off-axis optical elements is diamond turning.
Diamond turned optical surfaces contain grooves and ridges covering a broad range of spatial frequencies. Each spatial frequency component diffracts light at a characteristic angle increasing the stray light outside the desired small spot on the sample. This characteristic error produced by diamond turning is one of the main factors limiting the spatial dimension over which accurate optical metrology measurements of a sample can be made.
Off-axis mirrors made with conventional polishing [e.g. the technique used to make some large astronomical telescopes] would have less scatter and therefore perform better; however the process is very expensive. Replicated optics, characteristically molded in epoxy using a precision form, can be made cost-effectively; however, the performance and durability of these mirrors in optical metrology applications remain unproven.
It is another common practice in the prior art to employ catadioptric designs that incorporate off-axis spherical mirrors and refractive optical elements [e.g. U.S. Pat. No. 6,323,946]. Off-axis spherical mirror systems can be made with no central obscuration and can be finished by conventional polishing techniques; however, an off-axis spherical mirror can generate significant amounts of geometrical aberrations which enlarge the resultant focal spot. Mitigation requires the addition of auxiliary optics possessing equally large aberrations of opposite sign. This design form is highly stressed and is sensitive to manufacturing and alignment errors.
Refractive designs avoid the problems of reflective and catadioptric systems. However, because the refractive index of virtually all materials is wavelength dependent, refractive lenses exhibit chromatic aberration. It is difficult to develop broadband lens designs that are corrected for chromatic aberration over large spectral bandwidths. It is extremely difficult to develop large bandwidth, chromatically corrected designs that meet the requirements of small-spot optical wafer metrology systems.
In the prior art, U.S. Pat. No. 3,486,805 discloses a broadband photographic lens chromatically corrected over the wavelength range spanning 200 to 800 nm. The lens contains three elements: the first and third elements are positive lenses made of fluorite, the second element is a negative lens made from fused silica. However, the design is not appropriate for small-spot optical metrology where axial spot size over all wavelengths must be carefully controlled. The lens is optimized for photographic applications and cannot be scaled to the NA (xcx9c0.1) and focal length (xcx9c5 cm) appropriate to optical metrology without severe clipping of the beam.
U.S. Pat. No. 5,121,255 discloses a UV transmissive microscope objective. The objective contains a first lens group that includes a meniscus lens, made of quartz or fluorite, with positive power and a second lens group including a biconcave lens made of quartz and a biconvex lens made of fluorite. The second group also has positive power. The microscope objective is designed to produce small-spot illumination; however, since both first and second lens groups have positive power the chromatic aberration of the objective is difficult to minimize. Consequently, the objective is not suitable for small-spot, broadband wafer metrology applications. Further, these designs are unable to achieve the balance of axial spot over all wavelengths without an unacceptable level of sensitivity to misalignment or fabrication errors, or without adding more elements, which make them both more expensive, and also more prone to birefringence problems, which make them inappropriate for ellipsometric applications.
None of the prior art reflective, catadioptric and refractive designs are entirely satisfactory. It would therefore be desirable to develop a design for a high performance, broadband, refractive objective that is chromatically corrected over the wavelength range spanning 185-900 nm.
The assignee""s initial efforts to develop a refractive design are described in copending application Ser. No. 09/848733, filed May 3, 2001 and incorporated herein by reference. The refractive design described in the pending application was able to create a relatively small spot size on the sample, specifically, about 1.5 mm in diameter. While this design improved upon prior designs, further improvements are necessary in order to be able to focus into box sizes of 50 microns or less. The lens design described herein achieves this goal.
The subject of this invention is a family of refractive optical lens designs for focusing broadband light. The lens consists of three elements made from at least two different optical materials arranged in two groups. The first group includes a positive lens and a negative lens (in the preferred embodiment these lenses are made from different optical materials) with net negative power. The second group consists of a single bi-convex lens (in the preferred embodiment this lens is made from the same material as the positive lens in the first group). The lens elements are specified according to a prescription where the lens faces are consecutively designated from the front to the rear as the first to the sixth face, where rn is the radius of curvature of the nth face and tn is the distance between the nth and (n+1)st face. The elements of the lens system are arranged in a configuration where r2 less than F and the combined focusing power of the first lens group (first and second lenses) is negative. In addition, the spacing t4 between the first and second group of lenses is greater than 0.05F. This arrangement is optimized to produce a chromatically corrected focal spot with a focal spot diameter of less than about 50 microns over the wavelength region spanning 185 to 900 nm.
This family of optical designs is broadly applicable to a large class of broadband optical instruments commonly utilized in wafer metrology employing, spectrophotometry, spectroscopic reflectometry, spectroscopic ellipsometry and spectroscopic scatterometry techniques. The lenses systems can be used to focus light on a sample or to collect light from a sample (or both).